1. Field of the Invention
This invention relates to devices for converting television signals or other picture signals.
2. Description of the Related Art
The television signal of the NTSC system at the present time has 262.5 scanning lines for each field period or 1/60 sec. and, as the interlaced scanning is performed, 525 scanning lines for one frame. It is known to provide a picture signal converting circuit for increasing the number of scanning lines for each field of the television signal of the present day, for example, 2 times so that such a television signal is displayed on the high-resolution fine monitor or the like.
FIG. 1 illustrates the basic construction of this conventional picture signal conversion processing circuit.
As shown in FIG. 1, the television signal (analog signal) enters at an input terminal 1 and its high region is cut by a low pass filter (LPF) 2. It is then converted to a digital signal by an A/D converter 3. The output of the A/D converter 3 is applied to both of a first time axis compression circuit 5 and a field memory 4. The signal from the field memory 4 is applied to a second time axis compression circuit 6.
The output of the field memory 4 represents the signal for the preceding field which, in the case of the 2:1 interlaced scanning, traces the intermediates between the successive scanning lines of the present field. The time axes of such signals for the present and preceding frames from the A/D converter 3 and the field memory 4 are compressed to 1/2 by the time axis compression circuits 5 and 6 respectively. Then, their outputs are selected by a changeover switch 7 operating each time the scanning line of compressed time base is recycled so that the signals from both circuits 5 and 6 are taken out to produce a signal of the doubled number of scanning lines which is then applied to a D/A converter 8 which operates with a frequency equal to 2 times the sampling frequency of the aforesaid A/D converter 3. Further, the analog signal from this D/A converter 8 is made to pass through the LPF 9 which has a cut-off frequency equal to 2 times that of the aforesaid LPF 2. Thus, a high-resolution analog television signal of the doubled number of scanning lines is obtained at an output terminal 10.
The application of the above-described basic feature to the composite color television system is shown fundamentally in FIG. 2.
In FIG. 2, the composite color television signal from an input terminal 11 is separated to a luminance signal Y and a chrominance signal C by a Y/C separation circuit 12. The chrominance signal C is demodulated to color difference signals, for example, I and Q signals by a color demodulation circuit 13. The luminance signal Y is treated for an increase in the resolution (the doubled number of scanning lines) by a signal converting circuit 14 of the construction shown in FIG. 1.
Even for the color difference signals I and Q also, the same treatment as that for the luminance signal Y is carried out by a signal converting circuit 15. Along with the resolution-increased luminance signal from the circuit 14, they are converted to R, G and B signals for the three primary colors in passing through the matrix circuit 16, being displayed on the high-resolution color monitor 17.
In such scanning frequency doubling process, as shown in FIG. 3, a present field is formed in such a way that the intervals between the successive two scanning lines A and B of the present field are filled at interpolated positions X with the corresponding scanning lines X' of the preceding field i-1. That is, as the interpolating signal for the present field, use is made of the signal of the preceding field without any alternation.
With such a prior known technique, however, for a displayed picture of fast motion, the definition of the picture cannot always satisfactorily be obtained, although, when the motion is slow, the high resolution and high quality of the picture is assured on the display.
From this reason, there has been a previous proposal for making use, as the interpolating signal for the picture positions X, of an averaged signal of the corresponding upper and lower scanning lines A and B when the motion of the picture is fast. That is, as shown in FIG. 4, detecting means A is used to compare the difference between the signals for the preceding and next fields i-1 and i+1 with a fixed threshold value TH. When the speed of motion of the picture is determined to be faster than the prescribed limit, the output of a second signal forming means C which is a signal for interpolation of a present field i as was formed by using its own scanning lines is selected to be applied to an interpolating signal forming means D. When the motion is slow, the output of a first signal forming means B which is a signal for interpolation of the present field as was formed by using the signals of the preceding and next fields i-1 and i+1 is selected to be applied to that means D.
The prior known device of FIG. 4 is further explained in more detail. For note, the similar parts to those shown in FIG. 1 are denoted by the same reference characters. The analog television signal of the NTSC system from the input terminal 1 passes through the LPF 2 to the A/D converter 3. The signal from the A/D converter 3 is applied to a 262H (where H is the horizontal scan period) delay circuit 18 having an output which is connected to the input of a 1H delay circuit 19 having an output which is connected to the input of another 262H delay circuit 20.
Therefore, the scanning line signal X32 for the next field i+1 (produced directly from the A/D converter 3) is followed, after the delay of 262H, by the scanning line signal X23 for the present field from the delay circuit 18, then, after the further delay of 1H, by the scanning line signal X21 from the delay circuit 19, and then, after the further delay of 262H, by the scanning line signal X12 for the preceding field from the delay circuit 20, as shown in FIG. 5.
The outputs of the first 262H delay circuit 18 and the 1H delay circuit 19 are applied to an adder 21. The output of this adder 21 is applied to a 1/2 coefficient circuit 22. Hence, the 1/2 coefficient circuit 22 produces an output signal for interpolation in field of (X21+X23)/2.
Also, the outputs of the A/D converter 3 and the second 262H delay circuit 20 are applied to another adder 23. The output of this adder 23 is applied to another 1/2 coefficient circuit 24. Hence, the 1/2 coefficient circuit 24 produces an output signal for interpolation in between fields of (X12+X32)/2.
The outputs of these two 1/2 coefficient circuits 22 and 24 are selectively applied to the time base compression circuit 6 by the switch 25 in response to a control signal therefor as will be described more fully later. The output X21 of the 1H delay circuit 19 is applied to the time base compression circuit 5.
Meanwhile, in a subtractor 26, the difference between the frames is obtained in the form of a signal .alpha. representing the difference between the picture signal X12 and X32 for the preceding and next fields i-1 and i+1 as shown in FIG. 5. Then, the absolute value .vertline..alpha..vertline. of such a difference signal .alpha. is obtained by an absolute value circuit 28. The output of the absolute value circuit 28 is compared with a prescribed reference level TH by a comparator 27. And, when the output signal of the comparator 27 has "H" (high level), as the displayed picture is taken as motionless, the switch 25 is set to select the 1/2 coefficient circuit 24. When the output signal of the comparator 27 has "L" (low level), as the motion of the displayed picture is regarded as fast, the switch 25 is changed over to select the first 1/2 coefficient circuit 22.
However, in the type of device shown in FIG. 4, it is based on the difference between the signals for the preceding and next fields to the present field to be interpolated that the motion of the picture is detected. Therefore, if a rapid change of information has occurred in the signal for one field only, this cannot be detected. Thus, a problem of deteriorating the picture quality very seriously has arisen.
That is, because, in the above-described circuit, determination of which interpolating signal, (X21+X23)/2 for interpolation within the field, or (X12+X32)/2 for interpolation between the two fields, is to be selected is made depending on whether or not the absolute value of the difference signal between the preceding and next fields, or .vertline.X32-X12.vertline., is larger than the reference level TH, it results that even when the absolute value is, for example, smaller than the reference level TH to regard the displayed picture as a still picture, for, as a rapid motion and a sharp change in brightness are suddenly occurring only in the present field, the correlation of the present field with the preceding and next fields, that interpolating signal which has been obtained from the preceding and next fields, or (X12+X32)/2, is also selected to be used as the interpolating signal for the present field. Since this interpolating signal (X12+X32)/2 is based on the utilization of the correlation of the present field with the preceding and next fields, if the present field has little correlation with the preceding and next fields as has been described above, it is in the present field that no correlation is established between each of the scanning lines formed by the interpolating signal (hereinafter referred to as the "interpolated lines") and its adjacent upper or lower line. This constitutes the problem of lowering the picture quality largely. To eliminate this problem, therefore, it is advantageous to make use of second processing means so that the interpolating is performed within the individual field.